Method for monitoring gasket compression during fastener tensioning

ABSTRACT

A method for tensioning large bolts (16) used in securing covers (12) for openings in pressure vessels, pipe couplings, valves and the like, in which a sealing gasket (18) is used. The method is designed to prevent leakage in gasketed connections by monitoring the interaction of bolt tension and gasket compression. The method is particularly concerned with determining the bolt tension at which full gasket compression is achieved. This allows the user to verify proper gasket density, proper gasket seating, and proper bolt preload. The method involves measuring changes in a dimension (E) related to gasket compression as bolt tension (F) is increased. Full gasket compression to a metal compression stop is noted as a sharp change in the ratio ΔE/ΔF. A secondary method of identifying the point of full gasket compression utilizing acoustic transmission is described. The secondary method may be used to verify the results of the first method.

DESCRIPTION

1. Technical Field

The present invention is in the field of mechanical engineering and morespecifically relates to a method for monitoring gasket compressionduring bolt tensioning in pressurized fluid systems to prevent leakageof a fluid. Typical applications of the present invention would be onpiping systems used in nuclear power generating plants or in otherapplications with high internal pressures and where consequences ofleakage are severe.

2. Background Art

Piping systems, pressure vessels, pumps, and valves that contain highpressure and/or aggressive liquids or gasses are normally closed byflanged and bolted connections and are sealed by special gaskets.Traditionally, the bolts used in such devices have been tensioned by useof a torque wrench, but alternative methods are in use.

In one alternative method of tensioning the bolts, a special devicecalled a hydraulic tensioner is used. One type of hydraulic tensioneremploys hydraulic pressure to pull on the end portion of the bolt, andwith the bolt thus stretched, the nut, which is unloaded, is tightened.Bolt tensioners are described in the following U.S. Pat. Nos. 3,749,362;4,249,718; 4,438,901; and 4,433,828.

Although, in many cases, bolt tensioners are easier, more convenient,and more accurate to use than a torque wrench, such tensioners merelyapply force, and still include no apparatus or method for monitoring thecompression of the gasket.

In U.S. Pat. No. 3,643,501, Pauley describes a differentiator that turnsoff a power wrench when the tension applied to a fastener begins toexceed the elastic limit of the fastener. This range of tension is fargreater than that with which the present invention is concerned, andPauley's invention is based on a different physical effect than thepresent invention.

In U.S. Pat. No. 4,102,182, Brown, et al, describe a tensioningprocedure in which limits on the slope of the torque versus angle curveare employed.

In U.S. Pat. No. 4,400,785, Wallace, et al, use a microprocessor tomeasure successive areas under the torque versus angle curve todetermine whether a tightening criterion has been met.

In U.S. Pat. No. 4,228,576, Eshghy uses a torque or tension versus anglecurve to monitor or control tightening of fasteners.

None of the above patents provides a tensioning method that considersthe unique needs of pressure-sealing gaskets. In contrast, the presentinvention is concerned only with situations in which a gasket is to becompressed to a specific desired extent, and identifying the bolttension at which that specific gasket compression takes place.

DISCLOSURE OF INVENTION

The present invention is intended for use with a bolt tensioner onflanged and bolted connections of the type in which gasket compressionis limited by a metal compression stop. The compression stop may eitherbe part of the gasket or part of the flange. The invention permits theuser to determine the bolt tension at which proper gasket compressiontakes place, which allows the user to verify that the gasket was ofproper density and that the proper bolt preload was added.

Since high pressure sealing gaskets can be of different densities yetindistinguishable in size, shape, and color, a connection cannot beconfirmed as being properly tensioned without verifying proper gasketbehavior. If the gasket is too soft, the gasket will compress fully tothe compression stop with too little bolt tension, and may leakregardless of how much additional bolt tension is added. If the gasketis too dense, full gasket compression may not occur at maximum bolttension, leaving the full bolt tension on the gasket surface. In thislatter condition, future gasket relaxation reduces bolt tension and mayresult in a leak. This condition also allows the bolts to be subjectedto increased fatigue loading. To avoid leakage, full gasket compression(to the compression stop) must occur at proper bolt tension, withadditional bolt tension added to withstand variable internal andexternal loads. This leaves the gasket properly loaded, and, with thecompressing flanges rigidly connected metal-to-metal and adequatelypreloaded, joint movement and bolt fatigue loading are minimized.

Consequently, the major object of this invention is to monitor thecompression of the gasket as bolt tension is increased, and to detectthe tension at which full gasket compression, and thereforemetal-to-metal contact, is achieved.

Unlike prior art, the present invention directly measures thedisplacement produced by a specific amount of applied tension, andreveals the point of metal-to-metal flange contact by a sharp change inrate of displacement for a given increase in applied tension.

In some cases, when further verification of metal-to-metal contact isdesired, acoustic transmission (from one flange to the other) may bemonitored, with the changes in transmission associated withmetal-to-metal contact serving to confirm the previous indication.

The behavior of "bolts" and nuts is identical to "studs" and nuts forthe purposes of this invention. The appropriate choice sometimes isdictated by the component geometry. In further discussion, the terms"bolt" and "stud" will be used interchangeably, with the understandingthat one term may apply to the other as the application dictates. Theterm "threaded fastener" includes both "studs" and "bolts."

Sealing of connecting parts in high pressure connections is typicallyperformed with use of "spiral wound gaskets" of the type manufactured byFlexitallic Gasket Company, Inc., of Bellmawr, N.J.. The presentinvention is particularly well suited to industrial applications usingspiral wound gaskets.

In a preferred embodiment of the present invention, the user obtains avisual indication of the bolt tension at which proper gasket compressiontakes place and compares the information to established values todetermine the acceptability of the connection. In an alternativeembodiment, connection acceptability based on gasket behavior isdetermined and final fastener tension is achieved without interventionof the user.

In accordance with a preferred embodiment of the present invention, theseparation of the surfaces between which the gasket is compressed ismeasured as the applied tension is increased. When the gasket is beingcompressed, the separation decreases by a predictable amount for eachincrement of applied tension. However, after the gasket has beencompressed to the desired extent, metal-to-metal contact betweenportions of the opposing compressing surfaces occurs, and thereafter,further increases in tension result primarily in bolt deformation, withlittle effect on the separation of the opposing surfaces. Knowledge ofthe fastener tension at which this metal-to-metal contact occurs alongwith the final fastener tension is required to properly assess theacceptability of the connection.

In the most general form of the invention, any accurate means ofmeasuring changes in the separation (ΔE) of the compressing surfaces andchanges in fastener tension (ΔF) may be used to identify the transitionfrom the ratio ΔE/ΔF measured during gasket compression to thedistinctly different ratio ΔE/ΔF measured after full gasket compressionand resulting only from elastic deformation of the metal connectingparts. This permits measurements of separation to be made at convenientlocations on the device being tensioned or on the tensioning deviceitself, thereby facilitating use of the method of the present invention.It also permits the use of any of several known tensioning devices,which further enhances the usefulness of the method of the presentinvention.

The novel features which are believed to be characteristic of theinvention, both as to organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawings in which a preferred embodiment of the inventionis illustrated by way of example. It is to be expressly understood,however, that the drawings are for the purpose of illustration anddescription only and are not intended as a definition of the limits ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view partially in cross section showing avalve of the type of construction with which the method of the presentinvention may be used, and showing a means of measuring changes in theseparation of the compressing surfaces;

FIG. 2 is a side elevational view in cross section showing a pipecoupling of the type with which the method of the present invention maybe used, and showing a means of measuring changes in the separation ofthe compressing surfaces;

FIG. 3 is a side elevational view partially in cross section of apressure vessel with a manway and cover of the type with which thepresent invention may be used;

FIG. 4 is a side elevational view partially in cross section showing atypical hydraulic bolt tensioner in use.

FIG. 5 is a side elevational view partially in cross section showing atypical hydraulic bolt tensioner in use with a means of measuringdisplacement of compressing surfaces as a function of travel betweenparts of the tensioner.

FIG. 6 is a side elevational view partially in cross section showing atypical hydraulic bolt tensioner in use with a means of measuringdisplacement of compressing surfaces as a function of travel between theend of the threaded fastener and the outside of one of the compressingmembers.

FIG. 7 is a side elevation view partially in cross section showing amanway of the type with which the method of the present invention can beused including a secondary verification device;

FIG. 8 is a graph showing displacement as a function of applied bolttension as would be measured with the apparatus of FIG. 1;

FIG. 9 is a graph showing displacement as a function of applied bolttension as it would be measured with the apparatus of FIG. 5 and withthe apparatus of FIG. 6;

FIG. 10 is a block diagram showing a preferred embodiment of anapparatus implementing the method of the present invention;

FIG. 11 is a block diagram showing another way of implementing themethod of the present invention including secondary verification;

FIG. 12 is a block diagram showing another way of implementing themethod of the present invention;

FIG. 13 is a block diagram showing another embodiment of an apparatusimplementing the method of the present invention;

FIG. 14 is a block diagram showing a variation of the embodiment of FIG.11; and,

FIG. 15 is a flow chart showing the algorithm used in the embodiment ofFIGS. 13 and 14.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a check valve (flapper not shown) of the type on which themethod of the present invention could be used. The diameter of the flowpath will vary from several centimeters to a meter or more, and thevalve may weigh up to several thousand Kg. Access to the interior of thevalve for maintenance is provided by the lateral duct 10 which is closedby the bonnet 12. The bonnet 12 is secured to the body 14 of the valveby studs of which the stud 16 is typical.

A gasket 18 is compressed between the bonnet 12 and the body 14 to forma seal. Compression of the gasket 18 is intentionally limited by theannular land 22. The bonnet is thus shown in its sealing position.

During assembly, the gasket 18 is laid in place and the bonnet 12 isrested on it. Initially, the gasket 18 is only slightly compressed bythe weight of the bonnet, and the annular land 22 is not in contact withthe bonnet 12. Thereafter, the fasteners, of which the stud 16 istypical, are tensioned, drawing the bonnet ever closer to the body 14.The gasket 18 is gradually compressed until the land 22 makes contactwith the bonnet 12. This point at which the land contacts the bonnet isthe point at which proper gasket sealing has taken place; it is usuallyreferred to as the point at which "metal-to-metal contact" is reachedbetween the land and the bonnet. Further tensioning will do nothing toenhance the sealing of the gasket, but may be required to provideadequate preload to withstand internal and external forces.

As shown in FIG. 1, and in accordance with a preferred embodiment of thepresent invention, a linear variable differential transformer (LVDT) 24is mounted on a bracket 28 that is attached to the body 14. The LVDT 24includes a probe that makes contact with the bracket 26 that is mountedon the bonnet 12. In this way, the LVDT produces an electrical signal onthe lead wires 30 that is related to the separation between the bonnetand the body.

FIG. 8 is a graph showing the relation between the applied stud tensionF and the separation E₁ between the bonnet 12 and the body 14 of thecheck valve of FIG. 1. Upon assembly, initial values of stud tension Fand separation E₁ are shown as F_(i) and E_(i) respectively.

During the first phase of fastener tensioning, changes in E₁ representprimarily gasket compression plus some metal deformation, and theseparation E₁ decreases rapidly with increasing tension. However, atsome point 32 of FIG. 8, the land 22 of FIG. 1 makes contact with thebonnet 12. The stud tension at point 32 is shown as F_(m-m), andrepresents the tension at which "metal-to-metal" contact is developed.During tensioning after metal-to-metal contact has been obtained,changes in E₁ are slight, and represent metal deformation only.

FIG. 2 shows another situation in which the present invention can beused. FIG. 2 shows a coupling for joining two lengths of pipe. Thehalves 34 and 36 are drawn together by an arrangement of nuts and boltscomparable to that shown in FIG. 1.

It should be noted that the coupling of FIG. 2 lacks a featurecomparable to the annular land 22 of FIG. 1. To limit the compression ofgasket 38, a thinner ring 40 of metal is provided around thecircumference of the gasket 38. The ring 40, sometimes called acompression gauge or compression stop, effectively preventsover-compression of the gasket 38. The compression stop 40 is commonlysupplied as part of the gasket.

FIG. 3 shows another situation in which the present invention can beused. FIG. 3 shows a section of a pressure vessel wall 83, a manway 84and a circular manway cover 86. The manway 84 and the manway cover 86are drawn together by a number of nuts and studs situated around theperimeter of the cover, thereby compressing the gasket 19 as the studsare tensioned. As in FIG. 1, the gasket compression is limited to theproper amount by an annular land 23. Also similar to FIG. 1, FIG. 3 isshown with an LVDT 24 that produces an electrical signal that measureschanges to the value E₁ which is related to the separation of the manway84 and the manway cover 86.

One convenient apparatus for using the method of this invention is ahydraulic tensioner. One type of hydraulic tensioner is shown in FIG. 4,in which hydraulic force is used to compress the gasket and stretch thestud, allowing the nut to easily be run down to hold tension, afterwhich the hydraulic force is removed.

In the typical sample shown, a socket 42 fits over the nut 20 and isused to tighten the nut after the stud 16 has been stretched. The base44 fits over and surrounds the socket 42. The base 44 serves to positionthe other elements of the hydraulic tensioner. The base 44 includes anaperture 54 through which a tommy bar 52 may be inserted to rotate thesocket, and with it the nut. Other methods have been used to rotate thenut.

The hydraulic chamber housing 46 of the hydraulic tensioner sits on thebase 44 and includes a hydraulic chamber 60 in the form of a circulargroove. A ram 48 fits slidably within the hydraulic chamber 60, andsealingly engages it. The puller 50 is internally threaded for engagingthe threads of the stud 16, and when the puller has been screwed ontothe stud 16, it secures the ram, the hydraulic chamber housing, and thebase in the position shown.

Tensioning is accomplished by energizing a hydraulic pump 49. Thehydraulic pressure is transmitted through the duct 58 to the hydraulicfluid within the hydraulic chamber 60. The hydraulic pressure forces theram 48 against the puller 50, thereby stretching the stud 16. While thestud is in this stretched condition, the socket 42 is rotated to tightenthe nut 20 against the flange surface 88.

It is not necessary to apply any great torque to the nut 20, and inpractice it may be rotated manually until the nut makes firm contactwith the flange surface 88.

When the stud is being tensioned, the hydraulic pressure operates overthe constant area of the ram 48, and therefore, the force F applied tothe stud 16 is a function of the hydraulic pressure. Consequently, anelectronic pressure sensor 47 is used to determine the force F appliedto each stud. In some manual applications, a standard pressure gauge isused instead of an electronic pressure sensor.

In accordance with another embodiment of the present invention, as shownin FIG. 5, an LVDT 24 is mounted between brackets 26 and 28, andmeasures E₂, the movement between the stationary hydraulic chamberhousing 46 and the puller 50. Clearly, as the puller increases tensionto the stud 16, the dimension E₂ also increases.

In accordance with another embodiment of the present invention, as shownin FIG. 6, an LVDT 24 is mounted on the bracket 64, and the probeportion 62 of the LVDT extends to contact the end of the stud 16. TheLVDT measures changes in E₃, the distance between the top 88 of theflange surface and the stud 16. As the puller 50 increases tension tothe stud 16, the dimension E₃ also increases.

FIG. 9 shows the dimensions E₂ and E₃ as a function of the appliedtension F. Changes in E₂ and E₃ represent the cumulative effects ofgasket compression, metal deformation, and fastener elongation. Thedimensions E₂ and E₃ increase relatively rapidly as the gasket is beingcompressed, but when metal-to-metal contact is reached at point 32, therate of increase slows abruptly, and is limited to the deformation ofvarious metal parts including stud elongation. The degree of the changesin E₂ and E₃ as plotted in FIG. 9 differ from the degree of the changesin E₁ plotted in FIG. 8 in that stud elongation is not a factor in thearrangements plotted by FIG. 8.

The point 32 is the minimum tension required to fully seal the gasket,since further tension does not appreciably compress the gasket, butmerely applies additional preload to the connection. Once point 32 hasbeen reached, adequate preload may then be added to withstand variableinternal and external loads as required to minimize joint movement andfatigue loading on the bolts.

In accordance with a preferred embodiment of the invention, the point 32of FIGS. 8 and 9 can easily be recognized using the arrangement shown inFIG. 10. Through use of an LVDT, an electrical signal is produced whichrepresents the changes in one of the variables E₁, E₂ or E₃, denoted forsimplicity by E. Another electrical signal, F_(s), representing the bolttension, is produced by the bolt tensioner 66. The signal F_(s) may bederived from a pressure sensor of a hydraulic bolt tensioner. Thesesignals are applied to the vertical and horizontal axes, respectively,of the plotter 68 to produce graphs such as those shown in FIGS. 8 and9.

In accordance with the embodiment of the invention shown in FIG. 10, thetension is increased as the user watches the plotter 68. The userobserves the slope of the curve produced. For smaller values of F, theobserved slope should correspond to the slope calculated on the theorythat gasket is being compressed with limited metal deformation. The useris especially alert for changes in the slope. A sharp change in slope,shown as point 32 of FIGS. 8 and 9, indicates that metal-to-metalcontact has been reached. Further tensioning results primarily in metaldeformation and fastener elongation, and serves to preload theconnection. Once the predetermined desired preload has been added, theuser inhibits further increases in tension by the bolt tensioner, andtightens the nuts until they firmly contact the surface against whichthey bear. Thereafter, the user commands the bolt tensioner to relievethe hydraulic force altogether and removes the bolt tensioner from thebolt in question, leaving the nut to hold the connection at the desiredtension.

In some situations, it may be desirable to have secondary verificationof the bolt tension at full gasket compression. A method using acoustictransmission is used for this verification.

In accordance with the embodiment shown in FIG. 7, an acoustictransmitter 81 is placed on one flange and an acoustic pick-up 82 placedon the other. The acoustic devices are placed at maximum spacing fromthe fasteners and such that a direct acoustic path through thecompression stop will be developed when the gasket is fully compressedand the flanges are tensioned metal-to-metal.

In accordance with the embodiment shown in FIG. 11, acoustictransmission T from one flange to the other is plotted, along with E, asa function of bolt tension. The signal component corresponding to theacoustic pathway through the fasteners is filtered out. The remainingacoustic transmission is highly resistant to crossing gasket material oran air gap, and a sharp increase in transmission from one flange to theother flange occurs as the gasket is fully compressed and metal-to-metalcontact is developed. This sharp increase in acoustic transmissionserves to confirm that point 32 has been reached. Acoustic "throughtransmission" equipment of the type manufactured by Erdman IndustriesIncorporated of Pasadena, Calif., may be used for this application.

The use of acoustic transmission as a means of determining full gasketcompression is primarily considered a method of verifying the results ofthe method using measurements of E, since measurements of E provide amore complete picture of the interaction of the joint components.

FIG. 12 shows a variation of the embodiment shown in FIG. 10, in thatthe electric signal F_(S) related to bolt tension is obtained from anultrasonic extensometer 80 rather than the bolt tensioner. Such a deviceis manufactured by Raymond Engineering of Middletown, Conn.

FIGS. 13 and 15 show another embodiment of the present invention inwhich the bolt tensioner 66 is operated under control of a computer 70.In that embodiment, the tension applied by the bolt tensioner isincreased, and at uniform predetermined intervals (ΔF) tension increase,the dimension E is read by the LVDT 24, and is sent to the computer 70in the form of an electrical signal.

As shown in FIG. 15, this sensed value of E is stored in the computer,and tension is further increased. After the further increase by ΔF hasbeen accomplished, the next reading of E is read by the LVDT and isstored in the computer. The successive values of E are subtracted in thecomputer as indicated by the step 72 of FIG. 15. This calculatedincrement ΔE is then divided by ΔF to calculate the corresponding slopeM_(x) of the curve. This incremental slope M_(x) is compared to apredetermined stored value M_(B) and the magnitude of the difference isthen compared to a predetermined value d. The values "M_(B) " and "d"are described below.

The value of M_(B) is the slope of the portion of the curve that occursafter the gasket has been fully compressed and the flanges havecontacted metal-to-metal. The slope M_(B) is the result of deformationof the metal parts, and its expected value may be calculated.Alternatively, M_(B) may be empirically determined by pre-assembling theconnection without a gasket, or M_(B) may be taken from previous orsimilar assemblies. The expected value of M_(B) is initially stored inthe computer.

The preselected threshold level d provides a tolerance for thepredetermined M_(B) such that the value of M_(B) may be approximated,yet still easily identify the sharp change in slope noted as point 32 ofFIGS. 8 or 9. The test step 74 of FIG. 15 compares the value ΔE/ΔF withM_(B) after each increase in fastener tension, and is the computer'smethod of determining if the bolt tensioner is operating to the left ofthe point 32 of FIG. 8 or 9. In the event the tensioner is operating tothe left of the point 32, the computer commands the tensioner toincrease the bolt tension by ΔF, and the check is repeated. In the eventthat the computer determines that the bolt tensioner is operating to theright of the point 32 in FIG. 8 or 9, the program branches to the step78 in which the computer commands the bolt tensioner to increase thetension by the amount of the predetermined desired preload, F_(t).Thereafter, the nut is tightened and the tensioner force is relieved.

Although FIG. 13 shows the use of the bolt tensioner to provide thesignal F_(S), in a variation of that embodiment shown in FIG. 14, theultrasonic extensometer 80 is used to provide F_(S).

Normally, the bonnet 12 of FIG. 1, the coupling half 34 of FIG. 2, andthe manway cover 86 of FIG. 3 are secured by a number of studs. Incarrying out the procedure of the present invention, it is possible toprovide hydraulic tensioners of the type shown in FIG. 4 for use on someor all the studs simultaneously from a common pressurized hydraulicsupply.

Thus, there has been described a method for use with a bolt tensioner tomonitor the compression of a sealing gasket in order to identify thetension at which proper full gasket compression has been obtained. Amethod of secondary verification of full gasket compression is alsopresented. Further tensioning beyond this point may be required topreload the connection, which restricts joint movement and reducesfatigue loading of the bolts.

The foregoing detailed description is illustrative of severalembodiments of the invention, and it is to be understood that additionalembodiments thereof will be obvious to those skilled in the art. Theembodiments described herein together with those additional embodimentsare considered to be within the scope of the invention.

INDUSTRIAL APPLICABILITY

The method of the present invention is an improved way of makingclosures on pressurized fluid systems. The method allows the user tomonitor the closure for proper gasket behavior and therefore for properbolt preload. The method should find application in those industriesthat use pressurized liquids or gases where the consequences of a leakare very undesirable. Such installations include nuclear power plants,aerospace bases, refineries, chemical plants, and hydroelectric powerplants.

I claim:
 1. A method for determining the tension of a threaded fastener at which a specific limited amount of gasket compression takes place, the threaded fastener extending from a first member and passing through a clearance hole in a second member, a nut retaining the second member on the threaded fastener with an end portion of the threaded fastener extending beyond the nut, a gasket having a sealing portion included between opposing surfaces of the first and second members with the sealing portion being progressively compressed as the first and second members are drawn together but having compression mechanically limited to a desired amount, the method comprising the steps of:a) imposing a known force F between the second member and the end portion of the threaded fastener, the force F being in such direction as to draw together the first and second members; b) measuring a dimension signal E that is a function of the separation of the first and second members, such that changes in signal E indicate gasket compression and metal deformation; c) increasing the known force F by a known increment ΔF; d) measuring again the dimension signal E; e) determining the change ΔE in the dimension signal E resulting from the increase in step c) of the known force F; and, f) comparing the change ΔE with the change that would be expected if the first and second members had reached their mechanical limit of gasket compression.
 2. The method of claim 1 wherein an end portion of the threaded fastener extends beyond the nut and wherein a hydraulic tensioner is attached to the end portion of the threaded fastener and wherein the step of imposing a known force further comprises the steps of pressurizing the hydraulic tensioner and of measuring the hydraulic pressure in the hydraulic tensioner.
 3. The method of claim 1 wherein the step of applying a known force further comprises the step of rotating the nut.
 4. The method of claim 2 wherein the step of imposing a known force on the gasket further comprises the step of measuring the elongation of the threaded fastener.
 5. The method of claim 1 further comprising the additional and subsequent step of verifying through a secondary means that the first and second members had reached their mechanical limit of gasket compression.
 6. The method of claim 5 wherein an acoustical transmitter is attached to the first member for transmitting an acoustical signal, and an acoustical receiver is attached to the second member for receiving the transmitted acoustical signal, and wherein the step of verifying further comprises the step of monitoring the received acoustical signal for an abrupt change in its intensity which occurs when the first and second members reach their mechanical limit of gasket compression. 